System and method for fast focal length alterations

ABSTRACT

An apparatus and method for fast changing a focal length of a charged particle beam the method comprising the step of changing a control signal in response to a relationship between the control signal voltage value and the focal length of the charged particle beam.

RELATED APPLICATIONS

This application claims the benefit of U.S provisional application60/328,453 filed Oct. 10, 2001 titled “System and Method for fast focallength alterations”.

FIELD OF THE INVENTION

The invention relates generally to charged particle beam imagers, suchas scanning electron microscopes and the like and especially for asystem and method for fast focal length adjustments.

BACKGROUND OF THE INVENTION

The focal length of charged particle beams, such an electron beams, ionbeams and the like, is usually controlled by an objective lens being fedby a control current. Prior art objective lenses are characterized by arelatively large response period. For example, the frequency response ofa common objective lens has a cutoff at about 10 Hz. Accordingly; theobjective lens cannot be utilized for fast changes of the focal length.

There is a need to provide a system and method for fast adjustment ofthe focal length of a charged particle beam.

SUMMERY OF THE INVENTION

The invention provides a system and method for fast focal lengthchanges. The system and method control the focal length by changing theacceleration voltage. According to an aspect of the invention a primaryacceleration voltage is supplied by a stable power source, the primaryacceleration voltage is modulated by a modulation voltage to provide anacceleration voltage that is both relatively “clean” (high signal tonoise ratio) and stable and can also be changed very rapidly.Preferably, the stable power source is operative to provide primaryacceleration voltages that range between zero to hundreds, thousands andeven ten thousands of volts. The modulation voltage is supplied by acontrollable voltage supplier that supplies a modulating voltage thatranges between zero to one or two hundred volts.

According to an aspect of the invention the focal length system andmethod are utilized while imaging an object that is not perpendicular tothe charged particle beam axis (a “tilted” object”. The accelerationvoltage modulation tracks a scanning signal that controls the locationof the charged particle beam. Usually, the scanning signal of a tiltedobject resembles a periodic saw tooth signal. During each period thesignal first rises and then falls. A relatively large bandwidthcontrollable power supplier is required for accurately tracking therelatively sharp fall of the signal.

According to an aspect of the invention the focal length can be changedat rates that exceed TV rates and can be utilized for objects that aretilted at a large range of angles.

According to an aspect of the invention the focal length is changed atrates that far exceed the rates of implementing auto focus techniques.The invention provides a method for fast changing a focal length of acharged particle beam the method comprising the step of changing acontrol signal in response to a relationship between the control signalvoltage value and the focal length of the charged particle beam.

The invention provides a method for scanning a tilted object with acharged particle beam, the method comprising the steps of: providing acharged particle beam having a focal length that substantially matches apredefined scanning path of the tilted object, wherein the focal lengthis responsive to a control signal and wherein fast changes in the focallength are generated by fast adjustments of the control signal.

The invention provides a system for fast changes in a focal length of acharged particle beam, the system comprising: a charged particlegenerator for generating a charged particle beam having a focal lengththat is responsive to a control signal; a control signal generator forproviding a control signal for allowing fast changes of the focallength.

Conveniently, the control signal is an acceleration voltage, but othervoltages, currents, can be of use. Conveniently, the step of changing ispreceded by a step of calibration for determining the relationshipbetween the acceleration voltage value and the focal length of thecharged particle beam. The acceleration voltage comprises a primaryacceleration voltage and a modulation voltage. The charged particle beamis an electron beam. The charged particle beam is focused by anobjective lens and the fast changes are limited by a fast changing rate.The objective lens is operative to change the focal length of thecharged particle beam at a rate that is slower than the fast changingrate. The fast changes are usually limited by a fast changing rate andwherein the fast changing rate exceeds 10 hertz. Conveniently, fastchanging rate exceeds 10 kilohertz. Usually, the fast changes resultfrom scanning a tilted object. The tilted object is tilted at an anglethat does not exceed 45 degrees.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic image of a tilted inspected object, being imagedwithout utilizing fast focal length alterations;

FIG. 2 is a schematic illustration of a scanning pattern of an image, inaccordance with an aspect of the invention;

FIG. 3 is a schematic illustration of a scanning signal that may beutilized for imaging the image of FIG. 2, in accordance with an aspectof the invention;

FIG. 4 is a schematic illustration of a power supplies and electron gun,in accordance with an aspect of the invention; and

FIG. 5 is a side view of a calibration target, in accordance with anaspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Like every imaging device, the depth of focus (DOF) in Scanning ElectronMicroscope is limited. When the pixel size is smaller than the probesize, the DOF is limited by the numerical aperture of the objectivelens, whereas in case where the pixel size is greater than the probesize, the DOF is limited also by the distance between adjacent pixels.When the distance (also termed “working distance) between an inspectedobject and an imaging apparatus, such as a SEM, varies over theinspected object, (such as in the case of a tilted object) the maximumfield of view (FOV) that can be seen while keeping the sharpness similarall over the image, depends on the tilt angle.

It is known in the art that smaller acceleration voltages result insmaller the DOF. Experiment showed that the maximum FOV that can be seenwith the same sharpness over the whole image in tilt 45°, Vacc=600 V, isaround 5 microns. It means that the DOF in these conditions is ±1.75microns. As the search FOV for redetection is greater than 5 microns,redetection performance will degrade in tilt 45°. The image in FIG. 1shows an example of the focus problem phenomenon in tilt 45 degrees,when there is no dynamic focus correction. It is clear that both upperand lower sections of the image appear obscure, not in focus, while onlythe lines in the mid part are sharp.

On the other hand, the beam energy can be changed very quickly,therefore, it is a preferable method for correcting the focusdynamically in large tilt angles.

FIGS. 3 and 2 describe the dynamic focus ramp during the scanning periodin tilt. The ramp denotes the alternating electron beam energy (Vacc)during scan. FIG. 2 illustrates the scanning path of an electron beamthat is used to produce an image that is bounded by rectangular 12. Thehorizontal lines 13 (including the lines that are located outsiderectangular 12) illustrate the scanning path of an electron beam. Theoriented line 14 denotes the focal length.

FIG. 4 illustrates an exemplary arrangement of power suppliers. Thestable power supplies Vaccelerator, while the controllable power supplyis an alternating current power supply that supplies signal within therange of (0 to ±200 V).

Controllable power supply 30 is connected in series to Vaccelerator 32.Vaccelerator 32, Vextractor 34 and Vsupressor 38 are connected tovarious parts of an electron gun and to electrodes (such as 40 and 42),to provide a required voltage scheme for allowing fast changes in theDOF of the charged electron beam extracted from the electron gun 36.

According to an aspect of the invention a calibration step is performedprior to the scanning. The calibration umaps” modulation voltage valuesto focal length values. The calibration step is used to match modulationvoltage values to scanning patters.

The following is an exemplary calibration step that is utilized to matchbetween modulation voltage values and a scanning pattern of a 45° tiltedobject.${DynamicFocusRampAmplitude} = {\Delta\quad{{WD} \cdot \frac{\Delta\quad{Vacc}}{\Delta\quad z} \cdot {BitToVolt}}}$where,

-   -   1. ΔWD=0.7071·FOV    -   2.        ${\frac{\Delta\quad{Vacc}}{\Delta\quad z} = {a + {b \cdot {Vacc}}}},$        where Vacc is measured in kV.    -   a and b are a pair of constants. Its values depend on Vacc.

The parameter that needs to be calibrated is$\frac{\Delta\quad{Vacc}}{\Delta\quad z}.$

Actually, it is a measure of the chromatic aberration coefficient. Asthe chromatic aberration coefficient changes from column to column, onecannot put a fix value for this parameter, and it has to be measured.The easiest way of measuring $\frac{\Delta\quad{Vacc}}{\Delta\quad z}$is to deduce it from$\frac{\Delta\quad{OLC}}{\Delta\quad z}\quad{and}\quad{\frac{\Delta\quad{Vacc}}{\Delta\quad{OLC}}.}$$\frac{\Delta\quad{Vacc}}{\Delta\quad z} = {\frac{\Delta\quad{OLC}}{\Delta\quad z} \cdot \frac{\Delta\quad{Vacc}}{\Delta\quad{OLC}}}$$\frac{\Delta\quad{OLC}}{\Delta\quad z}$is measured on a step target containing steps of 50 microns heightdifference between each step.

Referring to FIG. 5, illustrating a calibration target that includesmultiple steps 51, 52, 53 . . . , that are spaced apart by 50 microns.If OLC1 (61) is the objective lens current that is used to focus thebeam on step 1 (51), and OLC3 is the objective lens current that is usedto focus the beam on step 3, and step 3 is 100 microns higher than step3,$\frac{\Delta\quad{OLC}}{\Delta\quad z} = \frac{{OLC1} - {OLC3}}{100\quad\mu\quad m}$$\frac{\Delta\quad{Vacc}}{\Delta\quad{OLC}}$is calculated by focusing on the same location in two different Vacc. IfOLC4 uses to focus in Vacc4 and OLC5 uses to focus in Vacc5, andVacc4−Vacc5=10 V,$\frac{\Delta\quad{Vacc}}{\Delta\quad{OLC}} = \frac{10\quad V}{{OLC5} - {OLC4}}$

1. A method for fast changing a focal length of a charged particle beamthe method comprising the step of changing a control signal in responseto a relationship between the control signal voltage value and the focallength of the charged particle beam.
 2. The method of step 1 wherein thecontrol signal is an acceleration voltage.
 3. The method of claim 2wherein the step of changing is preceded by a step of calibration fordetermining the relationship between the acceleration voltage value andthe focal length of the charged particle beam.
 4. The method of claim 2wherein the acceleration voltage comprises a primary accelerationvoltage and a modulation voltage.
 5. The method of claim 2 wherein thecharged particle beam is an electron beam.
 6. The method of claim 2wherein the charged particle beam is focused by an objective lens andwherein the fast changes are limited by a fast changing rate.
 7. Themethod of claim 6 wherein the objective lens is operative to change thefocal length of the charged particle beam at a rate that is slower thanthe fast changing rate.
 8. The method of claim 2 wherein fast changesare limited by a fast changing rate and wherein the fast changing rateexceeds 10 hertz.
 9. The method of claim 2 wherein fast changes arelimited by a fast changing rate and wherein the fast changing rateexceeds 10 kilohertz.
 10. The method of claim 2 wherein the fast changesresult from scanning a tilted object.
 11. The method of claim 10 whereinthe tilted object is tilted at an angle that does not exceed 45 degrees.12. A method for scanning a tilted object with a charged particle beam,the method comprising the steps of: providing a charged particle beamhaving a focal length that substantially matches a predefined scanningpath of the tilted object, wherein the focal length is responsive to acontrol signal and wherein fast changes in the focal length aregenerated by fast adjustments of the control signal.
 13. A system forfast changes in a focal length of a charged particle beam, the systemcomprising: a charged particle generator for generating a chargedparticle beam having a focal length that is responsive to a controlsignal; a control signal generator for providing a control signal forallowing fast changes of the focal length.
 14. The system of step 1wherein the control signal is an acceleration voltage.
 15. The system ofclaim 14 wherein the system is operative to determine a relationshipbetween the acceleration voltage values and the focal length.
 16. Thesystem of claim 14 wherein the acceleration voltage comprises a primaryacceleration voltage and a modulation voltage.
 17. The system of claim14 wherein the charged particle beam is an electron beam.
 18. The systemof claim 14 wherein the charged particle beam is focused by an objectivelens and wherein the fast changes are limited by a fast changing rate.19. The system of claim 18 wherein the objective lens is operative tochange the focal length of the charged particle beam at a rate that isslower than the fast changing rate.
 20. The system of claim 14 whereinfast changes are limited by a fast changing rate and wherein the fastchanging rate exceeds 10 hertz.
 21. The system of claim 14 wherein fastchanges are limited by a fast changing rate and wherein the fastchanging rate exceeds 10 kilohertz.
 22. The system of claim 14 whereinthe control signal provider comprises a stable power supply and a fastcontrollable power supply.